Light
Color
Effects
Transform
Border
Space: Compare · Ctrl+G: Grid · Ctrl+Z/Y: Undo/Redo
Presets
Layers
History
Artboards
How it works: the document is the board — it is bigger than the artboard, so you can add several images as layers and freely move them outside the artboard. When you Export, only the pixels inside the selected artboard are saved. Use the Artboard Tool (A) on the left toolbar — drag to create a new artboard, drag an artboard to move it, drag its corners to resize it.
Source
Use Use Current Image to split the picture you are editing, or drop your own file above.
Cut Lines
Click on the preview to add a horizontal cut. Drag a line to move it, or press × to remove it.
Output
Each part is auto-compressed under the size limit (quality is binary–searched, then the format is downgraded if needed).